1. Field of the Invention
The present invention broadly relates to the use of nondestructive testing for validating the integrity of a gas barrier member and, more particularly, is concerned with a method and apparatus for detecting surface tension screen failure using ultrasonics to measure variation in gas volumes on either side of the screen as a tank containing the screen is rotated.
2. Background of the Prior Art
A propellant tank for the Peacekeeper missile must be adapted to confine liquid propellant in such manner that a sufficient quantity of it will be available for delivery under zero gravity conditions. For this reason, the tank employs a propellant acquisition device (PAD) at one end which has a bulkhead containing several surface tension screens defining a holding chamber separated from the main chamber of the tank. A reasonably large ullage gas is present in the main chamber of the tank when the tank is in its fully loaded condition, while there is a minimal quantity of residual gas present in the PAD.
The bulkhead screens function in combination with the action of surface tension forces to maintain liquid propellant in the holding chamber at the delivery end of the tank and to replenish the holding chamber from the main chamber as the propellant is drawn from the holding chamber during firing of the missile. In order to serve their intended function, the screens must prevent gas flow between the main chamber and the PAD's holding chamber. The ability of gas to penetrate the screens will prevent the drawing of liquid propellant from the tank as the tank accelerates during missile firing.
The screens have a woven wire cloth construction and are rather delicate in nature. The primary desired characteristic of the screen is the ability to support a liquid column within the holding chamber through surface tension. Breakdown of meniscus between wires in the screen results in the flow of gas bubbles through the screen as the tank undergoes acceleration with the missile. In the case of screen failure, breaks in the wires of the metal cloth result in gas flow at a shorter liquid column. During ground handling of the tank when loaded with liquid propellant, high cycle fatigue and a single large flow impact on the screens may cause damage to them, allowing gas flow into the PAD. If a section of a screen dries out during storage, gas will also pass directly into the PAD from the main chamber. Further, during ascent of the missile, acceleration loads will force gas out of the PAD and back into the main chamber or upper section of the tank.
After the tank is assembled and filled, it is quite difficult to inspect the screens for bubble point degradation or damage due to ground handling. Therefore, a basic problem is to be able to test the assembled and partially filled tank to determine if the screens are damaged. The screens most susceptible to damage are those on the forward bulkhead of the PAD. Consequently, a need has arisen for a technique to validate surface tension screen integrity which does not compromise the pressure shell of the tank and which can be done at relatively low cost.